Testing the NXT Laser Target Sensor

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Testing my Mindstorms NXT LASER Target Sensor with 40mm P.V.C. Pipe Tubes wrapped with 3M Scotchlite Reflective 8910 Silver Fabric.

NXT LASER Target Sensor Test

I began testing my Mindstorms NXT LASER Target Sensor with 40mm P.V.C. Pipe Tubes  50mm Long wrapped with 3M Scotchlite 8910 Reflective Silver Fabric. The distance between the LASER Sensor and target was 1 Metre. The Sensor was also positioned so it was pointing straight a a North Facing Window which reaches down to floor level. I surmised that the light from the window would supply a constant bright ambient light level.

NXT LASER Target Sensor

With a Reflector too Sensor distance of 1 Metre (3′) the reading from the LASER Light Sensor in Reflected Mode pointed at the Target was ranged from a minimum value of 60% to a maximum of 75% depending on alignment. The smaller the angle of incidence, the greater the reflected component of the LASER Beam. In other words, if the LASER Beam struck the “Centre” of the Target Cylinder produced the maximum reading.

NXT LASER Target Sensor

When the LASER Beam was aligned as too miss the Target, the Sensor reading values remained below 57%. As I rotated through 360 degrees, the measured Reflected Light Value was 48%. Initially I thought I may have had to add a Red Filter which only allowed light to pass through a narrow frequency band either side of the LASER Beam Light Beam frequency would have to be placed in from of the Photo Transistor in the Sensor. The filter appears after testing at various ambient light levels not to be needed.

I’ve found that the LASER Sensor is able to detect the Reflective Cylinder at 1800mm (6′) with relative ease. Beyond this distance my reading were unreliable, which I think is an alignment issue due to me. The alignment jig I used when aligning the LASER Module was only around 1500mm (5′) between LASER Beam Source and the final position I used for the alignment card.

NXT LASER Targets

I’ve found that the easiest way to secure the 3M Reflective Tape is with very thin double-sided tape. Unlike liquid glue tpye adhesives, the tape doesn’t float about after positioning or before drying. Another alternative would be some spray-on type contact adhesive. I also constructed a couple of smaller Lego Panel type Reflectors which are great at shorter distances from the LASER Sensor.

LASER Reflection back on Sensor
Image showing the amazing amount of LASER Light Reflection Back to the Sensor from 450mm (1½’)

The Image shows the amazing amount of LASER Light Reflected Back to the Sensor from around 450mm (1½’) away from the reflector. I just managed to hold my F2 50mm Macro Lens still enough without any flash for the 1.5 seconds! Hence the slight graininess and very shallow depth of field. The ambient NXT Light Sensor reading was around 22%. When the LASER hits the Target from 1.8m (6′) away the Sensor Reads from 30% upwards. Here’s a list of Distance in Metres for the Reflected Beams Sensor Reading:

  • 2.0m = 32%
  • 1.5m = 34%
  • 1.0m = 36%
  • 0.5m = 45%
  • 0.3m = 48%
  • 0.2m = 52%
  • 0.1m = 55%

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